The present invention relates to a method and an apparatus for controlling the drafting of sliver in a draw frame in which sliver first travels through an advance sensing means that senses variations in the mass, then passes through a drafting means which is located after the advance sensing means and is provided with an adjustable draft and finally travels through an after sensing means located after the drafting means and which senses the mass per unit of length, whereby the draft of the drafting means is varied to compensate for variations in mass of sliver in response to the sensed variations in mass evaluated in a time-delayed manner and of the difference of the sensed mass per unit of length and of a selectable theoretical value for the mass per unit of length.
Known prior art is disclosed in German DE-PS 29 12 576. This prior art teaches that the absolute cross section of a sliver is sensed at the outlet of a draw frame and compared with a selected fixed theoretical value, and the relative cross-sectional variations of the sliver are also sensed at a location before the outlet of the draw frame in the direction of sliver travel. The average value of the relative cross-sectional variations is determined over a finite duration of time and compared with the instantaneous cross-sectional variations. A combined signal is formed from the sensed variations which controls regulating means for controlling the drafting means to attempt to draw the sliver to a uniform cross-section.
Additional prior art is disclosed in U.S. Patent Application Ser. No. 057,949, entitled A Method and Means for Controlling the Drafting of Sliver in a Draw Frame filed June 4, 1987, by the same applicants and owned by the same assignee as the present application. This application teaches a method and means for controlling the drafting of sliver in a draw frame in which sliver first travels past a sensing means that sense variations in the mass of sliver and then through a drafting means located after the sensing means. The draft can be changed in order to compensate for variations in mass, wherein a delay time in activation of the drafting means change is provided to account for the travel time of the sliver from the sensing means to the drafting means.
None of the prior art disclose the specific improvement of the present invention wherein a method and means for selecting a theoretical value for the mass per unit of length of sliver is provided, and the difference between this selected theoretical value and the mass sensed by an after drafting sensing means are determined. This difference is then combined with the time-delayed mass variation sensed by an advance sensing means to provide a correcting signal which can be applied to vary the drafting of sliver in response to the correcting signal. The improvements in precision and uniformity of sliver from the apparatus and method of the present invention will thus be readily perceived from the following.